The present invention relates to ovenable cartons prepared from paperboard, and more particularly to a venting and opening means for such a carton. The invention is directed to tray type cartons, or cartons which include a tray with an upper, integral, peripheral flange and a separate lid which is adhered or heat sealed to the flange to close the carton. This invention is further related to applicant's prior U.S. Pat. No. 5,244,145, entitled "Two Way Opening for a Tray Type Carton", granted Sep. 14, 1993.
Cartons of the type disclosed herein are well known in the art. They are low in cost, disposable and adapted to be used in the preparation of food, particularly with the use of a microwave oven. Such cartons are preferably constructed from a thermally stable paperboard material that is generally referred to as ovenable paperboard. For this purpose, the paperboard is coated on at least one surface with a heat sealable thermoplastic material, for example, a polyester material such as polyethylene terephthalate (PET). The coating also serves to make the paperboard waterproof and provides a food contact surface. However, when such cartons are heat sealed, the bond between the lid and the peripheral flange of the tray is difficult to break. Thus a variety of opening and venting schemes have been proposed for such cartons.
The most fundamental of such opening means has been the simple use of a knife or other sharp object for cutting the lid. Many cartons include dotted lines printed on the lid surface indicating where the lid can be cut for venting and opening. This has not proven to be a satisfactory method and a number of alternative means have been developed, including, as shown in U.S. Pat. No. 3,863,832, a bond between the flange and the lid that is made heat sensitive for failure at a pre-selected temperature. In addition a variety of other opening schemes are shown in other prior art patents such as U.S. Pat. No. 4,091,930, where cuts are applied at each corner to define tabs that may be pulled for removing the adjacent strip of cover and flange; U.S. Pat. No. 4,183,458, where partial depth cut lines are applied to the lid or flange adjacent to the lid-to-flange bond area which permits delamination of the lid/flange when a corner of the lid is pulled away from the tray; U.S. Pat. No. 4,531,668, where partial depth cut lines (microcuts) are applied to the inner and outer surfaces of the lid at a location inboard of the lid-to-flange bond area for opening the carton; U.S. Pat. No. 4,871,071, where the venting and opening scheme is applied only to the lid at one corner; and U.S. Pat. No. 4,955,530, where the venting and opening scheme is also applied only to the lid at one corner. These prior art opening and venting means are each satisfactory to some extent but they fail to address the problem solved by the present invention.